An initial investigation into claims that UK forces abused Iraqi detainees is to be opened by the International Criminal Court (ICC).

The ICC will analyse alleged crimes attributed to UK armed forces deployed in Iraq between 2003 and 2008.

Attorney General Dominic Grieve said the government "completely rejects" the claim that UK forces were responsible for systematic abuse.

This will be the first time the UK has been the subject of an ICC probe.

The head of the military prosecution body in the UK, Andrew Cayley, said it would co-operate.

But he said he believed it was unlikely that the ICC would push for a full, formal investigation, as the British government was already investigating claims of abuse in Iraq.

By Jonathan BealeDefence correspondent, BBC News

There'll be a mixture of emotions in government to today's news.

There'll be anger, frustration as well as a sense of embarrassment.

When Britain signed up to the International Criminal Court it would not have envisaged itself being the subject of any investigation - albeit the earliest "preliminary examination" stage.

Britain joins the likes of Afghanistan, the Central African Republic, Guinea and Georgia.

Even though this is not an ICC "formal investigation", at least yet, it's still a significant victory for the Human Rights Lawyer Phil Shiner and his firm Public Interest Lawyers.

He's long argued that British forces were involved in the systematic abuse of Iraqis.

He believes that mistreatment - some of which has been proven - was allowed by senior military officers and even ministers.

In other words that it wasn't just a "few bad apples".

The anger in the MOD and the military will be directed at Mr Shiner more than the ICC. They feel he's dragging the British military's reputation through the mud.

The decision by ICC prosecutor Fatou Bensouda to reopen the investigation, which was previously concluded in 2006, comes after a 250-page dossier of new information was submitted in January.

The dossier was submitted by the British law firm Public Interest Lawyers - headed by human rights lawyer Phil Shiner, who has been involved in a number of high-profile cases of allegations of British military personnel mistreating detainees in Iraq - and the European Centre for Constitutional and Human Rights, based in Germany.

It contained evidence of what they said was more than 400 cases of mistreatment or unlawful killings.

Among those named in the file are former defence secretary Geoff Hoon and former armed forces minister Adam Ingram.

The ICC in the Hague has jurisdiction over alleged crimes committed in Iraq by nationals of state parties under the Rome Statute.

'Highest standards'

During the preliminary examination, the prosecutor will consider issues of jurisdiction, admissibility and the interests of justice to decide whether to open a full investigation.

In a statement, the attorney general said he would co-operate fully with the ICC to demonstrate that "British justice is following its proper course".

He said: "The government completely rejects the allegation that there was systematic abuse carried out by the British armed forces in Iraq.

"British troops are some of the best in the world and we expect them to operate to the highest standards, in line with both domestic and international law.

British troops in Iraq: 2003 to 2011

UK armed forces joined US troops and other nations in an invasion of Iraq in March 2003

British forces peaked at 46,000 during the invasion phase

Levels then decreased year-on-year to 4,100 in May 2009 when the UK formally withdrew from Iraq

By the end of the eight-year war in 2011, around 44 UK troops remained in the country

The UK lost 179 servicemen and women during the war - 136 of those were killed in action

"In my experience the vast majority of our armed forces meet those expectations. Where allegations have been made that individuals may have broken those laws, they are being comprehensively investigated."

Mr Grieve added that he believed the work of inquiry teams to be "independent, robust and meticulous", and with the resources they need to do the job properly.

He said it was his job to ensure that continues to be the case.

"As the minister responsible for overseeing the UK's prosecutors, I understand the importance of the ICC prosecutor following the proper legal procedures when complaints are made," Mr Grieve said.

"The UK government has been, and remains a strong supporter of the ICC and I will provide the Office of the Prosecutor with whatever is necessary to demonstrate that British justice is following its proper course."

'Confident'

According to the Statute of Rome, which established the ICC, the court may only intervene in cases where there is no effective investigation being carried out by the national authorities.

Image copyrightAFP/GettyImage caption
A dossier submitted to the ICC, based in the building pictured, contains evidence of what lawyers say is more than 400 cases of mistreatment or unlawful killings

Mr Cayley, director of the Service Prosecuting Authority, said an inquiry was already under way through the Iraq Historical Allegations Team (IHAT), which was set up in 2010.

"If the ICC is satisfied that the United Kingdom is genuinely investigating these crimes, they will allow us to do that," Mr Cayley said.

"They may go on monitoring us for a number years in respect of investigations and prosecutions but they will not intervene."

He said it was for the prosecutor of the ICC to determine this, adding: "But I am confident based on the work that I've seen that IHAT has been doing, that the court will find that these are genuine criminal investigations that are taking place and they won't take it any further."